Recently, studies to reduce the size and weight of portable electronic devices, such as portable computers, portable telephones and camcorders, are being steadily conducted. In addition, secondary batteries, which are used as electricity sources for such electronic devices, require higher capacity, smaller size, lighter weight and thinner thickness. Among secondary batteries, following research and development of lithium-ion secondary batteries, such batteries are being produced and sold, because of their advantages, such as high voltage, long lifespan and high energy density.
The lithium-ion secondary battery is composed of a positive electrode, a negative electrode, a separator and an electrolyte solution. This battery has a structure of positive electrode-electrolyte solution-separator-electrolyte solution-negative electrode and exhibits battery characteristics not only by an electric charging process in which lithium ions migrate from the positive electrode to the negative electrode and are intercalated into the negative electrode, but also by an electric discharge process in which the lithium ions migrate from the negative electrode to the positive electrode.
Each of the electrodes (positive and negative electrodes) comprises a collector and an electrode film, in which the electrode film includes an electrode active material, a conducting agent and an electrode binder. Although an element which ultimately determines the properties of the electrodes is the electrode active material, the electrode binder which renders the electrode active material adhesive and serves to fix the electrode film to the collector also plays an important role.
For example, an electrode is prepared by a method comprised of uniformly dispersing an electrode active material, a conducting agent and an electrode binder in a solvent, to prepare electrode slurry, and then coating the electrode slurry on a collector. In this case, the dispersion and coating properties of the electrode slurry will be important.
A typical material for use as an electrode binder is polyvinylidene fluoride (PVDF) polymer which is used as a solution in an organic solvent, such as N-methyl-2-pyrrolidone (NMP). However, the PVDF binder causes an environmental problem resulting from the use of the NMP organic solvent and has a shortcoming in that it needs to be used at increased amounts for application to high-capacity secondary batteries. For this reason, studies on water-based binders, which can exhibit sufficient adhesion even at small amounts, are being conducted.
However, there is still no report on a water-based binder which can adjust the dispersion and coating properties of the electrode slurry.